1. Field of the Invention
The present invention relates to a gas-discharge display element. In particular, the present invention relates to such a gas-discharge display element having high luminance and high efficiency characteristics necessary for a display device which uses such elements and is capable of a tonal display.
2. Description of the Related Art
A gas-discharge display element or device is generally classified as an AC type and a DC type depending upon structure and driving system thereof. In either type, an electric discharge is generated in a discharge space around a selected image element thereof by applying, to gas filling a discharge space thereof, an electric field strong enough to start a discharge and Light emission is sustained by repeating such discharges.
FIG. 1A is a plan view of a conventional AC type gas-discharge display element and FIG. 1B is a cross section thereof taken along a line X-Y in FIG. 1A. The gas-discharge element includes a glass substrate 1, a pair of electrodes 4 formed thereon, a lamination of an Al.sub.2 O.sub.3 layer 5 and a MgO layer 6 covering the electrodes 4 and a front glass plate 10 opposing the lamination through a partition wall 7 provided on the lamination. On a lower surface of the glass 10, a write electrode 9 and an Al.sub.2 O.sub.3 layer 12 are formed in that order and a phosphor 8 is provided on a lower surface of the Al.sub.2 O.sub.3 layer 12 at a position opposing the electrodes 4.
In this AC type gas-discharge display element, an AC voltage having rectangular waveform is applied across the electrodes 4 arranged in a common plane and covered by dielectric member to generate and sustain an electric discharge between the electrodes. Phosphor 8 is excited by ultraviolet light produced in a discharge space 16 formed in an area defined by these electrodes 4 to provide a color display.
The generation and sustaining of the discharge is usually performed while an AC sustaining voltage pulse is supplied continuously from a power source 13 to the electrodes 4. To perform a writing, a write pulse is applied from a power source 15 to the electrode 9 in an interval between sustaining pulses of the sustaining pulse train or with the writing pulse added to the sustaining pulse. To perform an erasing, an erase pulse signal is applied from the power source 15 to the electrode 9 in the interval between the sustaining pulses. A memory mode operation of the element is performed by using same sustaining pulse within a time when the display element sustains light emission.
There are other AC type gas-discharge elements, such as a gas-discharge element operating not in memory mode but in line sequence mode and a gas-discharge element having a pair of electrodes 4 formed on opposing substrates.
In these pulse drive gas-discharge display elements, light emission intensity is modulated by controlling the number of pulses applied to selected electrodes within unit time to thereby control the number of light emissions.
Such an AC type gas-discharge display element has a sufficient memory function due to the fact that electric charge is accumulated in a dielectric layer on the electrodes. That is, a cell, once discharge is started, can sustain a discharge at a voltage Vs even lower than the discharge start voltage Vf. This memory capability can be very effectively used when the number of pixels or elements in a gas-discharge display panel as a display device is to be increased.
A substantial amount of ultraviolet light is produced by the gas-discharge in the gas-discharge display element with an early half duration of discharge current caused by gas-discharge. In order to utilize this highly efficient light emission, it is effective to drive the element with voltage pulses having small width. However, such drive with short pulses may cause an increase in the characteristic discharge voltage and a decrease of memory margin (Vf-Vs). When considering an improvement of element characteristics, these two effects appear simultaneously. Therefore, driving with short pulses is difficult and thus it is impossible to improve efficiency of light emission.
In FIG. 2, Vf is discharge start voltage, Vs is minimum discharge sustain voltage and M is memory coefficient (=(Vf-Vs)/2Vs) of a conventional element.
As mentioned, the conventional gas discharge display element is insufficient in light emitting characteristics, in particular, in efficiency of ultraviolet light emission for color display. Further, the modulation of light emission strength by controlling the number of emissions is not easy and it is very difficult to display multicolor tone.